General information
Kangaroos are marsupials and belong to the Family Macropodidae (i.e. big feet) that is grouped with the Potoroidae (potoroos, bettongs, rat-kangaroos) and Hypsiprymnodontidae (musky rat-kangaroo) in the Super-Family, Macropodoidea. This comprises around 50 species in
The Rock-wallabies (Petrogale spp.) is the most diverse genus amongst the living macropods with 16 species ranging from 1 to 12 kg in size. They are found across mainland Australia and on some recently separated offshore islands but not on the Bass Strait Islands, Tasmania or New Guinea. The species diversified from a common ancestor about 4 million years ago and their closest affinity to other macropods is with the Tree-kangaroos. Diversification of species occurred in two waves. The first gave rise to the Short-eared Rock-wallaby, the Monjon, the Narbelek, the Yellow-footed Rock-Wallaby and the Proserpine Rock-wallaby. The second was about a million years ago and lead to species that are not all morphologically distinctive like those along the Queensland seaboard. All Rock-wallabies favour habitat with rocky outcrops and slopes, cliffs and gorges or are found on boulder piles and escarpments especially in the wet-dry tropics. Their ability to scale precipitous rock faces in leaps that appear to defy gravity comes from adaptations to the feet and tail. The feet are short relative to the majority of macropods that inhabit flat ground. The pads are thick, spongy and highly granulated so that they compress on the rock surface and maximise grip. The tail is long and cylindrical with little taper and great flexibility. The tail acts as a counterbalance and rudder in rapid hopping across uneven surfaces and allows changes of direction in mid-air.
Species
Narbalek (Northern Territory)
Petrogale concinna concinna ('elegant rock-weasel')
Best place to see
Robin Falls, Northern Territory
Robin Falls is a small reserve along Dorat Road and about 18 km from the township of Adelaide River. The site is about 1.5 h drive south of Darwin. The falls cascade off sandstone escarpment which is bounded by floodplain to the east and the Adelaide River to the west. The area is predominantly private land and the home of rootourismTM is nearby. The falls are spring fed and water is present through the dry season and attracts other macropods like the Agile Wallaby, Antilopine Wallaroo at the base and on the cliff-faces, the Short-eared Rock-wallaby. The Narbalek can also be seen in western Arnhem Land and the Victoria River district.
Identification
Males and females average 1.3 kg in the dry season and 1.4 kg in the wet season. The maximum recorded weight is 1.7 kg. Thus the species is also known at the Little Rock-wallaby.
The fur is long and dense and the back is rufous overlain with white and dark brown to black marbling. The marbled appearance arises from longer hairs which have a dark brown base, a long white central band and a dark brown to black tip. The underfur is likewise dense and grey-white. The cheeks and head are grey with an orange tinge and the head is dissected by a darker dorsal stripe from between the eyes to the nape. There are indistinct broad stripes across the side of the face from the nose to the front of the eyes. The upper one is dark grey with an orange tinge and the lower one more yellow. There is some variation in the colour of the nape and shoulders from rufous to rust-red. The nose is naked and black. The fur is relatively sparse on the upper half of the ears and the inside of the ears is is defined by sparse white hairs. The arms, legs and feet are fawn merging to grey on the hands and feet. The undersides are yellow-grey. The tail gradually darkens towards the tip as longer dark brown to black hairs increase. About the distal third of the tail has a pronounced brush. When hopping they have the distinction of arching the tail over the back with the tuft prominent. This helps distinguish them from juveniles Short-eared Rock-wallabies with whom they may associate.
Habitat
The common habitat feature is not surprisingly rocks but these may vary from the cliffs of deeply fissured sandstone jump-ups (escarpments), to break-aways and isolated rock piles. The rocky habitat is embedded in similarly diverse tropical vegetation from floodplain, vine thickets, monsoon rainforest and open woodland. They may forage in the open hummock grasslands that adorn the slopes and plateaus of sandstone escarpments. All of these habitats are subject to the depredations of wildfire. Regimes of hot and frequent late dry season fires are challenging the long-term survival of this species. The once relatively pristine tropical savannahs of northern Australia have gone from an indigenous regime of 'caring for country' to one of 'caring for cattle' or 'don't care for anything but self-interest'. Arson is rampant and uncontrolled over much of this rock-wallabys' habitat, protective cover and food is burnt out, feral cats enter and prey heavily on residual wildlife, and the Narbalek's populations extinguish from year to year.
Foraging behaviour
You are unlikely to make a dental examination of a Narbalek but it is something of a shark amongst marsupials. It has an unlimited number of molars which shuffle forward along the tooth row as the frontal ones are worn down and lost (the analogy with sharks is this repetitive tooth replacement). The Narbalek early dispenses with its pre-molars and usually has four to six molars in play in each quadrant of the mouth. This unique dentition is an adaptation to a diet of highly silaceous (15-20% dry weight of silica) grasses and the fern Marsilea crenata at least on the floodplain sites where Narbalek diets were studied. You are more likely to see Narbaleks during the day in the Wet season when they may bask on the rocks in the morning and forage in the late afternoon. In the Dry season foraging is predominantly nocturnal and body condition may diminish as the seasonal drought hardens.
Narbaleks may forage with other rock-haunting macropods, especially Short-eared Rock Wallabies. The Narbalek is only the size of a juvenile of the latter species and is much faster and more agile.
Reproductive behaviour
Narbaleks breed throughout the year but are more likely to have a large pouch young or young-at-foot in the core of the Wet season (February) than the Dry season (August). In captivity, embryonic diapause has been demonstrated with gestation lasting 30-32 d and an oestrous cycle of 32-35 d with a post-partum oestrus. Pouch exit is at about six months and sexual maturity is achieved after a year.
Social organisation
The social organisation of Narbaleks is not well-described. They forage well out from rock shelters and may associate together and with other rock-wallaby species. The sexes intermingle on foraging sites.
Further readings
Churchill S (1997) Habitat use, distribution and conservation status of the Narbalek, P. concinna, and sympatric rock-dwelling mammals, in the Northern Territory. Australian Mammalogy 19, 297-308.
Nelson JE, Goldstone A (1986) Reproduction in Peradorcas concinna (Marsupialia: Macropodidae). Australian Wildlife Research 13, 501-505.
Pearson DJ, Kinnear JE (1997) A review of the distribution, status and conservation of rock-wallabies in Western Australia. Australian Mammalogy 19, 137-152.
Sanson GD, Nelson JE, Fell P (1985) Ecology of Peradorcas concinna in Arnhem Land in the wet and dry season. Proceedings of the Ecological Society of Australia 13, 69-72.